It's a thing that can focus intense ultrasound on a specific, programmable region. Really intense ultrasound can create bubbles that get very hot as they shrink and expand, and apparently you can use this to initiate polymerisation. Put the two together, and you can make a 3D printer.

Desirability is due to the fact that it's faster than printing with a single moving focus point, and sound can travel through things light or a physical printhead cannot, like a living person during surgery. Whether it actually pans out depends on all kinds of practical implementation details, but they make strong-sounding claims in the abstract of the paper.